The present invention relates to novel methods and apparatus for three dimensionally drawing polygons with filling and for drawing a desired figure over the polygons with accentuating.
In a graphics display apparatus, it is required that not only an object applied for display be merely displayed with filling but also that a unit polygon which constitutes an object be displayed with accentuating of its edge lines so as to easily visually recognize the size and control points and the like of the unit polygon.
When two dimensional polygons are displayed, edge lines of any of the polygons are easily displayed with accentuating and no disadvantage arises.
But when three dimensional polygons are displayed, a disadvantage arises in that the display of the edge lines having a predetermined width of the polygon with accentuating is remarkably difficult. More particularly, in the three dimensional graphics display apparatus, three dimensional feeling is generally represented by applying a hidden surface removal operation. When polygons are drawn with filling, depth data for each pixel of each polygon are calculated, then depth data for each pixel of a polygon are compared with depth data for corresponding pixels of other polygons, thereafter only necessary pixels are visually displayed. Now, when polygons are displayed with filling, depth data are interpolated in a scanning line direction, while when edge lines of a polygon are displayed with accentuating, depth data are interpolated in a edge line direction. Both depth data are not assured coincidence with one another. As a result, the disadvantage arises due to the lack of coincidence of both depth data that the disply quality of edge lines with accentuating is lowered, for example the edge lines may be displayed as dotted lines.
An apparatus is proposed to eliminate the disadvantage (Japanese Patent Laid Open Sho 63-15288). The apparatus first draws only edge lines of a polygon using a depth buffer algorithm and writes flag data into an edge-flag-memory which is used to control whether or not renewal of data stored in a depth buffer memory and a frame buffer memory is permitted. The apparatus then draws the polygon accentuating its edge lines with filling using the depth buffer algorithm and taking the flag data into consideration. The apparatus finally erases all flag data.
When the apparatus is employed, depth data and color data of the edge lines which are previously drawn are securely prevented from renewing despite the relationship between the interpolation operation results in the scanning direction and the interpolation operation results in the edge line direction. The disadvantage of the edge lines being displayed as dotted lines and the like is eliminated when the edge lines are displayed with accentuating for every polygon.
When polygons with their edge lines accentuated and regular polygons exist, depth data of edge lines may be different from depth data in the scanning line direction. The disadvantage arises that the edge lines are partially hidden so as to display the edge lines as dotted lines when regular polygons are drawn with filling using hidden surface removal after polygons with their edge lines accentuated are drawn with filling.
When polygons are displayed as translucent polygons, every polygon must be applied sorting in a depth direction (hereinafter referred to as z-sorting). In this case, depth data of edge portion of polygons with their edge lines accentuated and depth data of edge portion of regular polygons are mixed on a pixel basis, the former depth data may be greater for a pixel and the latter depth data may be greater for another pixel. The disadvantage arises in that the z-sorting of polygons becomes impossible.